Complain loudly when other conflicting plugins are installed

after/plugin/ale.vim

@@ -0,0 +1,28 @@
+if exists('g:loaded_ale_after')
+    finish
+endif
+
+let g:loaded_ale_after = 1
+
+if !g:ale_emit_conflict_warnings
+    finish
+endif
+
+function! s:GetConflictingPluginWarning(plugin_name) abort
+    return 'ALE conflicts with ' . a:plugin_name
+    \   . '. Uninstall it, or disable this warning with '
+    \   . '`let g:ale_emit_conflict_warnings = 0` in your vimrc file, '
+    \   . '*before* plugins are loaded.'
+endfunction
+
+if exists('g:loaded_syntastic_plugin')
+    throw s:GetConflictingPluginWarning('Syntastic')
+endif
+
+if exists('g:loaded_neomake')
+    throw s:GetConflictingPluginWarning('Neomake')
+endif
+
+if exists('g:loaded_validator_plugin')
+    throw s:GetConflictingPluginWarning('Validator')
+endif